Re: {Collins} 75S-3B Preselector Tuning Anomaly - "Miller effect?"


Suggest you measure the Xtal Osc and PTO output of the good receiver.
Then, compare those readings to the 75S-3B.

Your problem could very well be low signal injection from one or both of these oscillators.

On the 75S-3B, V-3 (Xtal Osc) or one section of V-4 (PTO cathode follower) could be weak/defective.
If these tubes are weak, the drive level into your 32S-3 will be affected.  This is especially noticeable on the 14MHz ranges and above.

Steve, N6HK

On November 5, 2017, at 15:12, Don Jackson via Collins <collins@xxxxxxxxxxxxx> wrote:


Certainly that apparently was a problem for some reason in the Heathkit. Yes, if the LO signal is being fed into the grid of V6, there could be some Miller Effect, even though it would be much less than if V6 were a triode. However, the Heathkit problem doesn’t apply to your problem if I understand you correctly.

Firstly, your problem follows a change of receivers, so the issue appears to be with the 75S-3B, not the input of the 32S-3 mixer. Secondly, the XTAL OSC is fed to the cathode of V5 in the 32S-3 2nd mixer, so the Miller Effect does not come into play anyway. The input impedance at the cathode of V5 should be relatively low, in the neighborhood of 100 Ohms.

Just some more thoughts.

Don, W5QN

From: jack m
Sent: Sunday, November 05, 2017 4:29 PM
To: Don Jackson ; collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 75S-3B Preselector Tuning Anomaly - "Miller effect?"


Here is an excerpt from the Heath mod for the SB-10x line of transceivers:

"The plate tank circuit of V6, the second transmitter mixer, is

used in the receive mode as the plate circuit of V10, RF

Amplifier. Due to tube Miller effects, additional capacity is

required when receiving to permit the driver preselector to

peak at the same dial setting in both transmit and receive

modes. This modification uses automatic diode switching to

add the required compensating capacity."

V6 is a pentode tube in the Heath rigs.

Perhaps I used the wrong analogy, or perhaps I should not have mentioned anything about Heathkit on this reflector, but it most closely defined what I was attempting to communicate with respect to what I was experiencing.  I beg the moderators' forgiveness if this is the case.

Thanks for your response, and I will look into your suggestions while on the bench.

Jack  W3RU


From: Don Jackson <w5qn@xxxxxxxxxxx>
Sent: Sunday, November 5, 2017 9:37 PM
To: jack m; collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 75S-3B Preselector Tuning Anomaly - "Miller effect?"

Hi Jack,

I'm not familiar with the "Miller Effect" in the Heathkit radios to which
you refer, but the Miller Effect is generally more of a problem with triode
tubes. The crystal oscillator tube in the 75S-3/3B is a pentode, so I doubt
that is the root of your problem.

The schematic diagrams of the XTAL OSC are identical in the 75S-3/3B, so
that shouldn't be an issue if the 75S-3B circuitry is working properly. I'd
start by terminating the 75S-3B XTAL OSC output port with a 100 Ohm
resistor, and making sure the amplitude level meets the requirements stated
in the manual. You do not mention if your problem is on all bands, so
measure the XTAL OSC output level on all bands. The VFO output from the
75S-3B should be measured as well to ensure it is functioning properly.

There is an article in the Q2 2012  issue of The Signal that discusses
possible problems with S-Line transceiver operation, but I doubt that your
issue fits those problems. Nevertheless, you can get some insight into the
cabling and impedance matching issues, if you have not already read the

Don, W5QN

-----Original Message----- .
From: jack m via Collins
Sent: Sunday, November 05, 2017 12:18 PM
To: collins@xxxxxxxxxxxxx
Subject: {Collins} 75S-3B Preselector Tuning Anomaly - "Miller effect?"

I currently experience a need to retune my 75S-3B preselector setting to
restore peak transmitter drive and/or receiver sensitivity when using my
32S-3 in transceiver mode.  This condition does not exist when the same
32S-3 is paired with my 75S-3.  The 75S-3B's sensitivity is significantly
degraded after this action.

All oscillator and RF-related inter-connecting cables are RG-62 coax about 4
feet in length, and the condition presents (or stops) with a straight
"switch-out" of the receivers.  It impacts my 30L-1 output from 600 to 400
ma and 500 to 400 watts output power, and the 32S-3 drive to the 30L-1 from
about 60 watts to 40 watts.  I have only tested for this using the Tune and
Lock Key modes, but not the CW mode.

I am most familiar with the need to install the Miller Effect mod in the
Heathkit SB-100/101 transceivers to correct this similar issue, but have not
found any reference to this effect in the archives to date.

I am most stumped by the absence of the condition when using the 75S-3, as
opposed to the -3B.  Before I take this unit to my bench for an alignment
check, I thought I would solicit input, possible explanations and advice.
Hopefully, it is not mechanical filter-based!

Thanks for any help.

Jack  W3RU
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